【Objective】The 15N tracing method is a key approach for quantifying gross soil nitrogen transformation rates and studying soil nitrogen transformation processes. Soil NO3--N is involved in multiple key processes, including mineralization, nitrification, denitrification, and dissimilatory nitrate reduction to ammonium (DNRA). Therefore, accurate determination of NO3--15N abundance is of great importance for understanding soil nitrogen cycling. The suspended diffusion method is simple to operate and cost-effective, and enables sequential separation and conversion of NH4+-N and NO3--N within the same system. However, it is still unclear how residual NH4+-N and reagent interferences introduce non-negligible errors in NO3--15N determination using this method. 【Method】A rapid method was developed to remove residual high-abundance NH4+-15N to minimize its interference with NO3--15N diffusion. The type of acid absorbent and the amounts of MgO and Devarda’s alloy were systematically optimized. The accuracy of the improved suspended diffusion method for determining labeled NO3--15N abundance was rigorously evaluated. 【Result】The results revealed that: (1) The N2 purging method was more effective in removing residual NH4+-15N than ultrasonic agitation and the conventional incubation-based approach. After 6 h of N2 purging, the residual high-abundance NH4+-15N (10.32 atom%) was completely removed, reducing the required diffusion time from ≥96 h to 54 h. This optimization also minimized interference from reagents and soil dissolved organic nitrogen (DON) decomposition. Under these optimized conditions, the difference between the measured NO3--N value and the theoretical value (0.370 0 atom%) was less than 0.003 8 atom%. (2) Using 1 mol·L-1 oxalic acid as the acid absorbent, together with 0.1 g MgO and 0.3 g Devarda’s alloy added, enabled the successful diffusion of both NH4+-N and NO3--N. After calibration, the difference between the measured value and the theoretical value was less than 0.140 0 atom%. (3) Also, there was no significant difference (P>0.05) in the determined soil labeled NO3--15N abundance between the optimized suspended diffusion method and the chemical conversion method or the denitrifier method, indicating that the improved technique yields equally reliable results. 【Conclusion】The optimized suspended diffusion method effectively eliminates interference from residual high-abundance NH4+-15N, significantly improving the precision and accuracy of NO3--15N determination. It also substantially shortens the time required for sequential NH??-N and NO3--N conversion within the same sample. This optimized diffusion method therefore provides important technical support for further research on soil nitrogen transformation processes and the global nitrogen cycle.